Abstract: The ability of homozygous Gunn rats to conjugate trimethylacetic acid with glucuronic acid was significantly reduced from that of the heterozygous Gunn rats. This was interpreted as indicative of a reduced glucuronosyl transferase activity in the livers of the homozygous rats. However, the glucuronosyl transferase which promote the synthesis of chondroitin sulfate chains in cartilages are apparently unaffected. Proteoglycans extracted from costal, tracheal and xiphoid cartilages of homozygous rats were indistinguishable from the proteoglycans isolated from the same cartilages of heterozygous rats. Indeed, the chondroitin sulfate chains from the cartilages of the homozygous rats were of the same size as those from the cartilages of the heterozygous rats.Comparison of the data obtained in this study with similar data on proteoglycans from bovine nasal septal cartilage revealed that the rat proteoglycan monomers were smaller than the bovine proteoglycan monomers. In addition, the protein cores of the rat proteoglycans had less histidine, arginine, proline, tyrosine, and phenylalanine and more serine and glycine residues than the protein core of the proteoglycan from bovine nasal septal cartilage.